The present invention relates to a high-speed, high-sensitivity and low-noise semiconductor photodetector for use in optical communication equipments, and more particularly to a high reliability planar avalanche photo-diode (hereinafter referred to as an APD) which is provided with a pn junction having what is called a guard ring and manufacturing method thereof.
It is well-known that an APD is very important as a high-speed, high sensitivity photo-detector needed in optical communication systems and the like. At present, a Si-APD formed of a Si crystal is widely employed in optical communications using a GaAs-AlGaAs laser of about 0.8 .mu.m wavelength. With a photo-detector using the Si crystal, however, it is difficult to detect light of a wavelength longer than 1 .mu.m, and it cannot be employed over a wavelength range of 1.0 to 1.7 .mu.m in which the transmission loss of silica fibers is low. As a photo detector sensitive to light of the 1 .mu.m wavelength band, there has been proposed a Ge-APD using a Ge crystal, but it is not satisfactory because of a large dark current and excess noise. Therefore, an APD which has high sensitivity characteristics in the 1 .mu.m wavelength range, using a semiconductor crystal of a III-V compound, is now being developed.
Materials for a photo-detector for the 1.0 to 1.7 .mu.m range are InGaAs, InGaAsP, AlGaAsSb, GaSb and so forth. A pn junction is formed in such a material and a reverse bias voltage is applied to the junction and, at the same time, light is applied to the photo detector from one side; thereby to detect a current resulting from photo-electric conversion. When a function of absorbing light to generate photo carriers and a function of multiplying the generated photo carriers are provide in a semiconductor layer of the same composition, since these semiconductor materials have a small forbidden band width, a generation-recombination current component and a tunnel current component, in the depletion layer increases upon application of the reverse bias, making it difficult to achieve low noise and high multiplication factor. With a view of overcoming the abovesaid defect, there has been proposed an APD of such a construction in which a light absorption region and a multiplication region are isolated from each other and the pn junction is provided in a different semiconductor of larger forbidden band, disposed adjacent the light absorbing layer, that is, a hetero structure APD (hereinafter referred to as an HAPD) as set forth in U.S. Pat. No. 4,274,103 and U.S. patent application no. 573,959 filed Feb. 21, 1984. For making the HAPD highly reliable, it is desired to employ a planar structure provided with a guard ring.
However, the guard ring of a conventional HAPD does not have sufficient functions as mentioned below.